Recently, an individual ignition coil type ignition coil for use in an engine has developed such an ignition coil is individually and directly connected to each of the ignition coils which are introduced to plug holes of the engine. In this kind of the ignition coil, a distributor becomes unnecessary, as a result at the distributor and a high tension cord for the distributor etc. a supply energy for the ignition coil does not fall down. In addition to these, without a consideration about a fall down of the ignition energy, it can design the ignition coil. Accordingly, a coil capacity can be made small and a small scale structure of the ignition coil can be devised, and further by an abolishment of the distributor, a rationalization of a component mounting space in an interior portion of an engine room can be devised.
In the above stated individual ignition type ignition coil, so as to mount the ignition coil by introducing at least a part of the ignition coil against to a plug hole, it is called as a plug hole coil. Further, so as to insert a coil portion to the plug hole, the ignition coil is called as a pencil type ignition coil which is long and thin in a pencil shape. This pencil type ignition coil has a center core (a magnetic core in which plurality of silicon steel sheets are laminated), a primary coil and a secondary coil at an interior portion of a long and narrow cylindrical shape coil case. The primary coil and the secondary coil are wounded to a respective bobbin and are arranged concentrically at a periphery of the center core. In the coil case for receiving the primary coil and the secondary coil, by potting and hardening an insulation resin and by filling up an insulation oil, thereby an insulation performance of the ignition coil is guaranteed. As the prior arts, for example, there are Japanese patent laid-open publication No. Hei 8-255719, Japanese patent laid-open publication No. Hei 9-7860, Japanese patent laid-open publication No. Hei 8-97,057, Japanese patent laid-open publication No. Hei 8-144916 and Japanese patent laid-open publication No. Hei 8-203757. Further, in the pencil type ignition coil, there is taken into a consideration in which to restraint the leakage fluxes passing an outer periphery of the coil a side core is provided at the outer periphery of the coil case.
In the pencil type ignition coil, there is two types, one of them in which the primary coil is arranged at an inner side and the secondary coil is arranged at an outer side, and another of them in which the secondary coil is arranged at an inner side and the primary coil is arranged at an outer side. A latter type (a structure of secondary wire is arranged inside primary wire) has an advantage merit about an output characteristic in comparison with a former type (a structure of secondary wire is arranged outside primary wire).
Namely, in case of the pencil type ignition coil in which an insulation resin (for example, an epoxy resin) is potted and hardened to a coil constitution member, as shown in FIG. 7, in the structure in which the secondary wire is arranged outside the primary wire, the primary coil, the epoxy resin, a secondary bobbin, the secondary coil, the epoxy resin, a coil case, and a side core are provided from the inner side in order. In this structure, an electrostatic floating capacitance generates between the secondary coil and the primary coil which is arranged at an inner side of the secondary coil and has a low voltage (this is regarded as a substantial ground voltage), and further an electrostatic floating capacitance generates between the secondary coil and the side core (a ground voltage). As a result, in comparison with the structure in which the secondary wire is arranged inside the primary wire, the electrostatic floating capacitance of the side core follows superfluous, accordingly the electrostatic floating capacitance of the structure in which the secondary wire is arranged outside the primary wire tends to become large. (On the other hand, in the structure in which the secondary wire is arranged inside the primary wire, an electrostatic floating capacitance generates between the secondary coil and the primary coil, and between the primary coil and the side core both the primary coil and the side core has the ground voltages, the electrostatic floating capacitance does not generate substantially).
A secondary voltage output and a secondary voltage rising speed are affected by the electrostatic floating capacitance and the more the electrostatic floating capacity becomes large, the more the output lowers and a delay in the rising generates. As a result, the structure having the small electrostatic floating capacitance in which the secondary wire is arranged inside the primary wire is considered to suit for a small scale structure and a high output performance.
In the case of the structure in which the secondary wire is arranged inside the primary wire, in the structure between the secondary bobbin and the center core, it is an important problem that how an anti-heat shock performance and a mitigation of electric field concentration are compatible with.
The above stated secondary bobbin has a role of an insulation of a high voltage generated in the secondary coil from the center core. In a case where a gap is provided between the secondary bobbin and the center core, a difference in an electric field strength (an electric field strength of a gap portion becomes extremely large, an electric field concentration) generates, a dielectric break down generates at the gap portion between the secondary coil and the center core. To prevent the dielectric break down, it is necessary to fill up an insulation member between the secondary bobbin and the center core and to mitigate the electric field concentration.
However, in the case where the resin is filled up between the secondary bobbin and the center core, according to a difference between the coefficient of linear thermal expansion (13.times.10.sup.-6 mm/.degree.C.) of the center core and the coefficient of linear thermal expansion of the resin, there is an axioms that cracks cause in the resin and the dielectric break down generates. As such a crack prevention countermeasure, it is conceivable that by blending a silica filler etc. the coefficient of linear thermal expansion of the resin approaches to that of the center core. However, in the above case, a flowability of a resin molding lowers and in particularly there is a problem that it is difficult to pot the resin to a gap (one figure level mm at a decimal point) between the center core and the secondary bobbin which is a minute clearance.
Then the inventors of the present invention have devised a method in which a flexible epoxy resin having a glass transition point at less than a normal temperature (20.degree. C.) and young's modulus of 1.times.10.sup.8 (Pa) at more than the normal temperature was filled up between the secondary bobbin and the center core. (For example, Japanese patent application No. Hei 7-326800, Japanese patent application No. Hei 8-249733). Herein, the flexible epoxy resin is defined as a soft epoxy resin which has a soft state at the normal temperature. Such a soft epoxy resin is injected, for example, under a vacuum condition to get extremely rid of voids (a vacuum potting type).
The soft epoxy resin has the superior anti-heat shock performance (the heat shock absorption, the heat shock mitigation) against to a repeated thermal stress since the soft epoxy resin has an elasticity. By an employment of the above stated soft epoxy resin, the heat shock against to the center core and the heat shock against to the secondary bobbin can be mitigated and further by an employment of the material having a superior adhesion performance, it can prevent the clearance occurrence between the center core and the secondary bobbin, but on the other hand since an insulation performance is low in comparison with a bobbin material, it is desirable to make thin to the utmost and a thickness of the second bobbin is assured and then the insulation performance between the secondary coil and the center core.
Objects of the present invention are that
(1) An object of the present invention is that, in an individual ignition type ignition coil (for example, a plug hole coil) in which the above stated secondary wire being arranged inside the primary wire structure is employed and is led into a plug hole, an anti-heat shock performance and a relaxation of electric field concentration (an insulation performance) between a secondary coil and a center coil can be improved and a quality (a reliability) and a working productivity in manufacturing can be heightened.
(2) Another object is that, even in an engine having a plastic cylinder head cover, an individual ignition type ignition coil can be adopted without any obstacle and a light weight structure of the engine can be realized.